Programmed cell death plays a critical role in regulating cell turnover during embryogenesis, metamorphosis, tissue homeostasis, viral infections, and cancer. Previously, we identified and cloned three mammalian genes encoding inhibitor of apoptosis proteins (IAPs): HIAP1, HIAP2, and XIAP (Farahani, R., et al, Genomics, 42:514-8, 1997; Liston, P., ct al., Genomics, 46:495-503, 1997a; Liston, P., et al., Nature, 379:349-53, 1996). While the IAP genes were initially discovered in baculoviruses, their homologues have since been identified in other viruses, insects, birds, and mammals, suggesting a common evolutionary origin.
X-linked IAP (XIAP) is a member of the mammalian IAP gene family. The anti-apoptotic function of XIAP is executed, at least in part, by inhibition of caspase-3 and caspase-7, two principal effectors of apoptosis. Interestingly, XIAP mRNAs are present in all human and murine fetal and adult tissues examined.
Most eukaryotic mRNAs are translated primarily by ribosome scanning. First, the 40S ribosomal subunit with its associated initiation factors binds to the 5'7-methylguanosine (m.sup.7 G)-cap structure of the mRNA to be translated. The complex then scans in the 3' direction until an initiation codon in a favorable context is encountered, at which point protein translation is initiated. According to this model, the presence of a 5' untranslated region (UTR) with strong secondary structure and numerous initiation codons would present a significant obstacle, leading to inefficient translation by ribosome scanning. Ribosome reinitiation, shunting, and internal ribosome binding are secondary mechanisms of translation initiation that alleviate the requirement for ribosome scanning and allow translation to proceed in a cap-independent manner.
Internal ribosome entry site (IRES) elements, which were first identified in picornaviruses, are considered the paradigm for cap-independent translation. The 5' UTRs of all picornaviruses are long and mediate translational initiation by directly recruiting and binding ribosomes, thereby circumventing the initial cap-binding step.
Although IRES elements are frequently found in viral mRNAs, they are rarely found in non-viral mRNAs. To date, the non-viral mRNAs shown to contain functional IRES elements in their respective 5' UTRs include those encoding immunoglobulin heavy chain binding protein (BiP) (Macejak, D. G., et al. Nature, 35390-4, 1991); Drosophila Antennapedia (Oh, S. K., et al., Genes Dev, 6:1643-53, 1992) and Ultrabithorax (Ye, X., et al., Mol. Cell Biol., 17:1714-21, 1997); fibroblast growth factor 2 (Vagner, S., et al., Mol. Cell Biol., 15:35-44, 1995); initiation factor eIF4G (Gan, et al., J. Biol. Chem., 273:5006-12, 1998); proto-oncogene c-myc (Nanbru, et al., J. Biol. Chem., 272:32061-6, 1995; Stoneley, M., Oncogene, 16:423-8, 1998); and vascular endothelial growth factor (VEGF) (Stein, I., et al., Mol. Cell Biol., 18:3112-9, 1998).
Cellular IRES elements have no obvious sequence or structural similarity to picomavirus IRES sequences, or to each other. Moreover, the mechanism for the regulation of IRES-directed translation is not understood. An understanding of the mechanism by which IRES elements direct cap-independent translation of cellular mRNAs and characterization of novel IRES sequences will provide new approaches for regulating the intracellular levels of both endogenously- and exogenously-encoded proteins.